Modern petroleum drilling and production operations demand a great quantity of information, relating to parameters and conditions downhole. Such information typically includes characteristics of the earth formations traversed by the borehole, along with data relating to the size and configuration of the borehole itself. The collection of information relating to conditions downhole, which commonly is referred to as “logging”, can be performed by several methods.
In conventional wireline logging, a probe (or “sonde”) containing formation sensors is lowered into the borehole after some or all of the well has been drilled. The formation sensors are used to determine certain characteristics of the formations traversed by the borehole. The upper end of the sonde is attached to a conductive wireline that suspends the sonde in the borehole. Power is transmitted to the instruments in the sonde through the conductive wireline. Conversely, the instruments in the sonde communicate information to the surface using electrical signals transmitted through the wireline.
An alternative method of logging is the collection of data during the drilling process. Collecting and processing data during the drilling process eliminates the necessity of removing the drilling assembly to insert a wireline logging tool. It consequently allows the driller to make accurate modifications or corrections as needed to optimize performance while minimizing down time. “Measurement-while-drilling” (MWD) is the term for measuring conditions downhole concerning the movement and location of the drilling assembly while the drilling continues. “Logging-while-drilling” (LWD) is the term for similar techniques, which concentrate more on the measurement of formation parameters. While distinctions between MWD and LWD may exist, the terms MWD and LWD often are used interchangeably. For the purposes of this disclosure, the term LWD will be used with the understanding that this term encompasses both the collection of formation parameters and the collection of information relating to the movement and position of the drilling assembly.
In LWD systems, sensors typically are located at the lower end of the drill string. More specifically, the downhole sensors are typically positioned in a cylindrical drill collar positioned near the drill bit. While drilling is in progress these sensors continuously or intermittently monitor predetermined drilling parameters and formation data and transmit the information to a surface detector by some form of telemetry. Alternatively, the data can be stored while the sensors are downhole, and recovered at the surface later when the drill string is retrieved.
Once drilling on a well has been completed, the well may be used for production of hydrocarbons. The well bore may be lined with casing to prevent collapse. The casing may be perforated in certain regions to permit hydrocarbons to enter the well bore from the formation. A string of production tubing may be lowered through the casing to where the hydrocarbons are entering the well bore. Particularly in the situation where the casing is perforated at multiple levels, instruments may be attached to the production tubing to determine the location, type and amount of hydrocarbons that enter the well bore. The instruments may additionally be configured to perform control operations to limit or enhance flows in selected regions of the well bore.
In addition, or alternatively, completed wells may be used for seismic data gathering and long term reservoir monitoring. Typically, an array of sensors is disposed along the length of a well and fixed in place. A telemetry system gathers the sensor data into a central (surface) facility where the data may be processed to extract desired information.
As drilling technology improves, deeper wells are drilled. Pressures and temperatures become significantly higher at greater well depths. At temperatures approaching 200 Celsius, the performance of existing electronic technologies degrades or fails. It would be desirable to create data acquisition systems that are suitable for use at temperatures approaching and well in excess of 200 C.